1. Field of the Invention
The present invention relates to techniques for color toning. More specifically, the present invention relates to a method and an apparatus for generating a set of color-toning curves which can be used to color tone an image.
2. Related Art
The term “black-and-white” is usually a misnomer when applied to monochrome prints. Since pure (or neutral) “black-and-white” images usually look “lifeless,” artists like to impart a color to monochrome images. For example, an artist might want to warm the highlight regions (making them more orange) in an image while cooling the shadows (making them more blue). This process of imparting a color to a monochrome image is called “color toning.” Note that color toning (or casting) can also be applied to color images.
There are a number of prior art techniques for color toning. One approach uses multi-tone curves which specify particular amounts of ink to be used for particular input gray levels. These inks then combine to produce a color-toned image when printed. For example, if one wanted to create an image with cooler shadows and warmer highlights, one could build a tri-tone combining some combination of black ink applied in proportion to the gray level together with an application of blue ink only in the shadows and orange ink only in the highlights. Another color toning approach uses gradient maps which specify mappings for the colors in an image and essentially provides a way to specify the application of color in particular regions of an image. Yet another approach is based on color balancing. In this approach, a split tone effect is generated by adjusting the color balance in the highlights and shadows.
Unfortunately, these prior art techniques have a number of disadvantages. First, the techniques are cumbersome to use. Typically a user has to manually specify the color toning curves. To do this, the user may need to go through multiple trial and error iterations before obtaining the correct color toning curve. Second, the techniques may generate relatively harsh toning curves that clip off portions of the range in order to achieve their effects. Third, these techniques can lead to undesirable side effects. For example, prior art techniques may cause shadows to become darker and highlights to become brighter, which is usually undesirable.
Hence, what is needed is a method and an apparatus for color toning which is easy to use and which generates smooth transitions between the various color tones without causing undesirable side effects.